1. Field of the Invention
The invention relates to an inverter, a motor control apparatus, and a power steering system using the motor control apparatus.
2. Description of the Related Art
A power steering system performs steering assistance by transmitting a driving force of a motor to a steering shaft. An inverter is used to control such a motor. The inverter includes a bridge circuit having a plurality of sets each of an upper arm and a lower arm connected together in series between a positive electrode and a negative electrode of a DC power supply such that the sets are connected together in parallel. A middle point between the upper and lower arms of each set in the inverter is connected to a corresponding phase coil of the motor. The upper arm and the lower arm each include a switching element. The switching elements are controllably turned on and off in accordance with control signals. On-off actions of the switching elements allow phase voltages to be supplied to the corresponding phase coils of the motor. The motor is thus rotated to generate a driving force.
As the DC power supply, for example, a replaceable battery is used. When replacing the battery, a user may erroneously connect the positive electrode and the negative electrode of the battery to a negative electrode side and a positive electrode side, respectively, of the inverter. Thus, measures are taken to allow no current to flow through the inverter if the positive electrode and negative electrode of the battery are connected to the negative electrode side and positive electrode side, respectively of the inverter.
In an example of such a measure to prevent back-flow, a mechanical relay is inserted between the inverter and the DC power supply. In this case, when the DC power supply is reversely connected, the mechanical relay makes the circuit between the inverter and the DC power supply open. Alternatively, a diode or an FET may be inserted between the inverter and the DC power supply to allow no current to flow when the DC power supply is reversely connected.
By way of example, Japanese Patent Application Publication No. 2010-114957 (JP 2010-114957 A) discloses a configuration in which a MOSFET for interruption in a non-use state is connected in series between the positive electrode of the DC power supply and the inverter. When the positive electrode and negative electrode of the DC power supply are connected to the negative electrode side and positive electrode side, respectively, of the inverter, output from the reversely connected DC power supply keeps the MOSFET for interruption in the non-use state off. This allows prevention of a possible short circuit between the terminals of the DC power supply when the DC power supply is reversely connected.
The insertion of the mechanical relay as in the above-described related art may lead to an increased size of the apparatus. The insertion of the diode or FET may increase a resistance component of a current path (for example, a voltage drop in the case of the diode). This may increase losses to the apparatus.